As downsizing of an AT-cut crystal resonator proceeds, it has become difficult to fabricate crystal elements for crystal resonators by a fabrication method of mechanical processing. Then, an AT-cut crystal element fabricated using photolithography technique and wet etching technique has been developed.
A known AT-cut crystal element and crystal resonator fabricated by the above-described techniques is disclosed in, for example, Patent Documents 1 and 2. Specifically, Patent Document 1 discloses an AT-cut crystal element in which a side surface (Z′-surface) intersecting with a Z′-axis of crystal is constituted of two surfaces of an m-surface of a quartz crystal and a crystal surface other than the m-surface, and a crystal resonator that employs the AT-cut crystal element. Patent Document 2 discloses an AT-cut crystal element in which a side surface intersecting with a Z′-axis of the crystal (Z′-surface) is constituted of at least four surfaces, and a crystal resonator that employs the AT-cut crystal element.
In each case of Patent Documents 1 and 2, an etching resist mask for forming an outer shape is formed on a quartz substrate, and a part on the quartz substrate without being covered with the mask is dissolved by the wet etching. Specifically, in the case of Patent Document 1, the wet etching is performed to the quartz substrate such that two surfaces including the m-surface are formed on the corresponding side surface. In the case of Patent Document 2, an etching of forming the outer shape is firstly performed to the quartz substrate on which the etching resist mask is formed. Then, the etching resist mask is removed to expose the quartz substrate, and subsequently, the wet etching is performed to the quartz substrate such that at least four surfaces are formed on the corresponding side surface. In each case of Patent Documents 1 and 2, the etching resist mask is formed to be used on the quartz substrate while being displaced on the front and back of the quartz substrate in a Z′-direction by a predetermined amount (a mask-displacement amount Δz). The mask-displacement amount Δz is appropriately Δz=0.75×T±20% when a thickness of the AT-cut quartz substrate is indicated as T (μm). Each crystal element formed in such process is reduced vibration leakage to achieve an AT-cut crystal resonator with excellent property.